Lioness at Indianapolis Zoo kills her longtime mate

This undated photo provided by the Indianapolis Zoo shows the lion Zuri at the zoo in Indianapolis. The zoo said Friday, Oct. 19, 2018, that Zuri, attacked male Nyack in their outdoor yard Monday before the zoo opened for the day. A necropsy confirmed Nyack died of suffocation from injuries to his neck. (Indianapolis Zoo via AP)

This undated photo provided by the Indianapolis Zoo shows the lion Nyack at the zoo in Indianapolis. The zoo said Friday, Oct. 19, 2018, that an adult female, Zuri, attacked male Nyack in their outdoor yard Monday before the zoo opened for the day. A necropsy confirmed Nyack died of suffocation from injuries to his neck. (Indianapolis Zoo via AP)

INDIANAPOLIS — The Indianapolis Zoo is trying to determine why a lioness killed her mate.

The zoo said Friday that an adult female, Zuri, attacked 10-year-old male Nyack in their outdoor yard Monday before the zoo opened for the day.

The zoo says the two had been housed together for eight years and produced three cubs in 2015. Nyack was on loan from the San Diego Zoo.

It says that staff heard an unusual amount of roaring from the yard and tried to separate the two but that Zuri held Nyack by the neck until he stopped moving. A necropsy confirmed Nyack died of suffocation from injuries to his neck.

The zoo says daily logs kept by staff previously showed no unusual aggression, injuries or wounds between the two.